In the current context of increasing transmission bit rates and the explosive growth of Internet services, many broadband access systems have been made available to home users. These high bit rate access systems use existing cables, i.e. twisted-pair copper cables. High bit rates providing users with high-speed and audiovisual Internet services can be transported on existing copper pairs of an analog public switched telephone network (PSTN) or an integrated services digital network (ISDN) using x-DSL digital coding techniques. The term “x-DSL” refers to all broadband high bit rate services combining all technology families, such as HDSL, SDSL, ADSL, VDSL, ADSL-Lite, etc.
However, x-DSL systems that enable voice and data to be conveyed on the same medium must be able to adapt to different copper-wired terminal infrastructure topologies (star, bridge, or parallel line termination sections, line lengths, etc.), support all dynamic variations of transmission characteristics, and co-exist with narrowband (analog or ISDN) voice band services.
This requirement applies essentially to systems operating at very high frequencies, typically in a band above 3 MHz, for example VDSL systems, which have a very wide spectrum extending from 138 kHz to 12 MHz. The use of a transmission channel in a range of high frequencies is not without consequences in respect of the quality of the connection. In particular, transmission in the band above 3 MHz is sensitive to reflections on the line caused by open branches and/or capacitive loads.
The expression “open branch” refers to any unconnected line termination section, i.e. any unoccupied telephone jack. The expression “capacitive load” refers to any line termination section connected via a telephone jack to a narrowband terminal representing a capacitive load, for example a telephone.
In the context of international standardization, the ETSI standard includes two VDSL frequency plans, known as 997 and 998, having incompatible spectra. The first provides more room for the uplink channel and favors symmetrical high bit rates (26 Mbit/s in the downlink direction and 26 Mbit/s maximum in the uplink direction), whereas the second favors asymmetrical bit rates with a higher bit rate on the downlink channel (34 Mbit/s on the downlink channel and 4 Mbit/s maximum on the uplink channel).
The invention is described below with reference to the prior art:                FIG. 1 is a diagram of a prior art copper-wired terminal installation including a single filter;        FIG. 2 is a diagram of a prior art copper-wired terminal installation including distributed filters.        
A copper-wired terminal installation (ITC) can have either of two types of configuration. It can have a “private” configuration, of “house” type, or a “collective” configuration, of “apartment block” type. The present invention applies without distinction to both configuration types.
A conventional VDSL connection comprises a multiport DSL access multiplexer (DSLAM) and a user modem that are respectively installed in the exchange and in a copper-wired terminal installation (ITC) on the user premises and are interconnected via an access network RA. The maximum line length is approximately 1 kilometer (km). In the case of an “FTTCab” architecture, the DSLAM is shifted from the exchange to a subdistribution frame and the exchange and the DSLAM are connected via optical fiber. Those architectures are known to the person skilled in the art and are not described in more detail here.
The VDSL connection must also be transparent to the telephone channel. On the user premises, the entry point of the ITC is an input terminal strip, also known as a network interface device (NID), and the filter function for separating the voice band from broadband services depends on which of the two types of installation shown in FIGS. 1 and 2 is used.
In the first case (FIG. 1 installation), the ITC comprises a splitter comprising a single filter FU at the entry point to the home, immediately after the NID. This single filter FU filters the lower portion of the spectrum. Thus the splitter separates the voice band, which is passed to narrowband terminals TBE1, TBE2, such as telephones, from broadband services, which are passed to a broadband terminal TBL, such as a computer, for example, via a high bit rate VDSL modem M in this particular example. This type of filter FU is generally a high-order (n>5) filter, which necessitates intervention of the telephone operator to install it on the user premises.
In the second case (FIG. 2 installation), the ITC comprises distributed filters FD1, FD2. These filters are passive microfilters, generally second order filters, which can be plugged into the telephone jack ahead of the narrowband terminals TBE1, TBE2. These filters prevent broadband signals interfering with telephone signals and vice-versa. They are low-cost components that are easily fitted by users themselves. However, their number cannot be increased indefinitely since their resultant impedance can compromise the return loss values, which could degrade voice quality on the telephone connection. Moreover, in the case of an ITC with distributed filters, high bit rate transmission in the 3-12 MHz band is sensitive to reflections on the line in the presence of open branches BO1, BO2 and/or capacitive loads TBE1, TBE2.
This problem could be avoided by inserting a terminating impedance into each jack to favor broadband transmission at the same time as reducing attenuation phenomena caused by mismatching of the telephone line impedance. A terminating impedance of this kind preferably has a value close to the mean value of the impedance of the copper cable of the telephone line in the frequency band from 3 to 12 MHz, that is to say a value of the order of 135 ohms (Ω). However, if a modem, for example a VDSL modem, is connected to a telephone jack of this kind into which a terminating impedance has previously been inserted, the performance of the high bit rate connection is degraded because the jack is then not transparent to the transmission of broadband services in that the value of the impedance previously inserted into it is close to that of the modem (of the order of 135 Ω in the 3-12 MHz band).
One solution to the problem of degraded high bit rate connection quality in the 3-12 MHz band caused by mismatching of the line impedance would be to match the impedance of the open branches by inserting a terminating impedance into the telephone jacks and removing the impedance previously inserted into a jack as soon as a modem is connected thereto, to prevent degrading the performance of the high bit rate connection. This solution is far from the ideal, however, as it is entirely manual and obliges the user to remember to remove an impedance previously inserted into a jack before plugging in a high bit rate modem.